<!doctype html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Multi Billiard 1</title>
    <link rel="stylesheet" href="../include/style.css">
  </head>
  <body>
    <header>
      Example from <a href="http://amzn.com/1430236655?tag=html5anim-20"><em>Foundation HTML5 Animation with JavaScript</em></a>
    </header>
    <canvas id="canvas" width="400" height="400"></canvas>

    <script src="../include/utils.js"></script>
    <script src="./classes/ball.js"></script>
    <script>
    window.onload = function () {
      var canvas = document.getElementById('canvas'),
          context = canvas.getContext('2d'),
          balls = [],
          numBalls = 8,
          bounce = -1.0;

      for (var radius, ball, i = 0; i < numBalls; i++) {
        radius = Math.random() * 20 + 15;
        ball = new Ball(radius);
        ball.mass = radius;
        ball.x = i * 100;
        ball.y = i * 50;
        ball.vx = Math.random() * 10 - 5;
        ball.vy = Math.random() * 10 - 5;
        balls.push(ball);
      }

      function rotate (x, y, sin, cos, reverse) {
        return {
          x: (reverse) ? (x * cos + y * sin) : (x * cos - y * sin),
          y: (reverse) ? (y * cos - x * sin) : (y * cos + x * sin)
        };
      }

      function checkCollision (ball0, ball1) {
        var dx = ball1.x - ball0.x,
            dy = ball1.y - ball0.y,
            dist = Math.sqrt(dx * dx + dy * dy);
        //collision handling code here
        if (dist < ball0.radius + ball1.radius) {
          //calculate angle, sine, and cosine
          var angle = Math.atan2(dy, dx),
              sin = Math.sin(angle),
              cos = Math.cos(angle),
              //rotate ball0's position
              pos0 = {x: 0, y: 0}, //point
              //rotate ball1's position
              pos1 = rotate(dx, dy, sin, cos, true),
              //rotate ball0's velocity
              vel0 = rotate(ball0.vx, ball0.vy, sin, cos, true),
              //rotate ball1's velocity
              vel1 = rotate(ball1.vx, ball1.vy, sin, cos, true),
              //collision reaction
              vxTotal = vel0.x - vel1.x;
          vel0.x = ((ball0.mass - ball1.mass) * vel0.x + 2 * ball1.mass * vel1.x) /
                   (ball0.mass + ball1.mass);
          vel1.x = vxTotal + vel0.x;
          //update position
          pos0.x += vel0.x;
          pos1.x += vel1.x;
          //rotate positions back
          var pos0F = rotate(pos0.x, pos0.y, sin, cos, false),
              pos1F = rotate(pos1.x, pos1.y, sin, cos, false);
          //adjust positions to actual screen positions
          ball1.x = ball0.x + pos1F.x;
          ball1.y = ball0.y + pos1F.y;
          ball0.x = ball0.x + pos0F.x;
          ball0.y = ball0.y + pos0F.y;
          //rotate velocities back
          var vel0F = rotate(vel0.x, vel0.y, sin, cos, false),
              vel1F = rotate(vel1.x, vel1.y, sin, cos, false);
          ball0.vx = vel0F.x;
          ball0.vy = vel0F.y;
          ball1.vx = vel1F.x;
          ball1.vy = vel1F.y;
        }
      }

      function checkWalls (ball) {
        if (ball.x + ball.radius > canvas.width) {
          ball.x = canvas.width - ball.radius;
          ball.vx *= bounce;
        } else if (ball.x - ball.radius < 0) {
          ball.x = ball.radius;
          ball.vx *= bounce;
        }
        if (ball.y + ball.radius > canvas.height) {
          ball.y = canvas.height - ball.radius;
          ball.vy *= bounce;
        } else if (ball.y - ball.radius < 0) {
          ball.y = ball.radius;
          ball.vy *= bounce;
        }
      }

      function move (ball) {
        ball.x += ball.vx;
        ball.y += ball.vy;
        checkWalls(ball);
      }

      function draw (ball) {
        ball.draw(context);
      }

      (function drawFrame () {
        window.requestAnimationFrame(drawFrame, canvas);
        context.clearRect(0, 0, canvas.width, canvas.height);

        balls.forEach(move);
        for (var ballA, i = 0, len = numBalls - 1; i < len; i++) {
          ballA = balls[i];
          for (var ballB, j = i + 1; j < numBalls; j++) {
            ballB = balls[j];
            checkCollision(ballA, ballB);
          }
        }
        balls.forEach(draw);
      }());
    };
    </script>
  </body>
</html>
